1. Field of the Invention
This invention relates to electromagnetic beam formation and transmission/reception systems, and more particularly to radar systems employing active transmit/receive (T/R) modules for localized transmission and reception functions.
2. Description of the Prior Art
To achieve a radar system with a high volumetric coverage without mechanically moving the antenna, antenna arrays in the past have been designed in circular, cylindrical and spherical configurations. Many of these configurations have used antenna feed systems, with a centralized RF power source and various types of constrained feed or space feed power distribution schemes. Examples of centralized feed systems are described in Antenna Engineering Handbook, 2nd Edition, Richard C. Johnson, Henry Jasik, Editors McGraw-Hill Book Co., New York, 1984, pages 21-12 through 21-21.
The centralized feed systems all require some way to efficiently distribute the RF power to the antenna radiating elements. For a constrained feed system, dividers and combiners are required that can introduce undesirable inter-element interference and losses. With space feed systems, difficult problems arise from a lack of aperture efficiency, and interference such as jamming and spurious radiation. With either approach, the entire system is subject to catastrophic failure in the event of a loss of the centralized power source.
More recently, active T/R modules have been developed that make it possible to generate RF power directly at the antenna element, to set relative phase relationships between the elements, and to perform pre-amplification of the received signal, all within the active T/R module. Locating the modules at each antenna element in the antenna array simplifies the problem of activating non-linear/non-planar array configurations without a central RF power source.
Active T/R module arrays have low RF losses, a lower vulnerability to interference, and distributed rather than centralized RF power generation. However, the need to place an active T/R module at each element of the antenna array adds significantly to the production cost, and also to the weight, of a radar system that uses this type of active array. Cost and weight penalties are incurred even if only a portion of the array is activated at any given time.